A suppressor, sound suppressor, or sound moderator; also called silencer, is a device attached to or part of the barrel of a firearm which reduces the amount of noise and also usually the amount of muzzle flash generated by firing the weapon. Suppressors can be used both with gunpowder-based weapons and with compressed air weapons.

A suppressor is usually a metalcylinder with internal mechanisms to reduce the sound of firing by slowing the escaping propellant gas and sometimes by reducing the velocity of the bullet

Early suppressors were created around the beginning of the 20th century by several inventors. American inventor Hiram Percy Maxim (not to be confused with Hiram Stevens Maxim, Hiram Percy's father, and the inventor of the Maxim Machine Gun), is credited with inventing and selling the first commercially successful models circa 1902 (patented 30 March 1909). Maxim gave his device the trademarked name Maxim Silencer. The muffler for internal combustion engines was developed in parallel with the firearm suppressor by Maxim in the early 20th century, using many of the same techniques to provide quieter-running engines (in many European countries automobile mufflers are still called silencers). The word silencer in reference to gun suppressor remains in widespread use, although it has since fallen out of favor with some, especially among the firearms industry, being replaced with the more literally accurate term sound suppressor or just suppressor, because a "sound suppressor" does not "silence" any weapon, rather it eliminates muzzle flash and reduces the sonic pressure of a firearm discharging. Common usage and U.S. legislative language favor the historically earlier term, silencer. In U.S. law, the terms "firearm muffler" and "firearm silencer" are synonymous.

Suppressors were regularly used by agents of the United StatesOffice of Strategic Services, who favored the newly designed High Standard HDM.22 Long Rifle pistol during World War II. OSS Director William Joseph "Wild Bill" Donovan demonstrated the pistol for PresidentFranklin D. Roosevelt at the White House. According to OSS research chief Stanley Lovell, Donovan (an old and trusted friend of the President) was waved into the Oval Office, where Roosevelt was dictating a letter. While Roosevelt finished his message, Donovan turned his back and fired ten shots into a sandbag he had brought with him, announced what he had done and handed the smoking gun to the astonished president. The British SOE (Special Operations Executive) Welrod assassination pistol with an integral suppressor was also used by the American OSS on clandestine operations in Nazi occupied Europe during the Second World War.